For example, 2-3 pulldown conversion processing is used as a technique for converting 24 frame-per-second progressive signals which are the same as film pictures used in recording movies and so forth (24P signals) into 30 frame-per-second interlaced (i.e., 60 field-per-second) signals (60I signals).
As shown in FIG. 1, 2-3 pulldown conversion processing is processing wherein four frames of 24P signals prior to 2-3 pulldown (2-3 pd) are converted into ten fields (five frames) of 60I signals, which is repeated as a sequence, thereby converting 24P signals into 60I signals.
Shown at the upper portion in the example in FIG. 1 are four frames (one sequence) of 24P signals before 2-3 pulldown, along the passage of time t, and below the frames are illustrated ten fields (one sequence) of 60I signals following 2-3 pulldown. The Ts and Bs assigned to the ten fields indicate top fields and bottom fields, respectively.
That is to say, due to 2-3 pulldown conversion (hereafter also simply referred to as 2-3 pulldown), the first frame from the left in 24P signals is converted into the first top field from the left and the second bottom field from the left in the 60I signals. The second frame from the left in 24P signals is converted into the third top field from the left, the fourth bottom field from the left, and fifth top field from the left in the 60I signals. The third frame from the left in 24P signals is converted into the sixth bottom field from the left and the seventh top field from the left in the 60I signals. The fourth frame from the left in 24P signals is converted into the eighth bottom field from the left, the ninth top field from the left, and tenth bottom field from the left in the 60I signals. This conversion processing is repeated as one sequence, as illustrated at the bottom in FIG. 1.
Now, in order to differentiate between cases wherein a set of two fields is converted from a frame and cases wherein a set of three fields is converted from a frame in order to facilitate description, the lower portion of FIG. 1 illustrates the above-described one frame of 24 signals with two squares arrayed vertically, the top field and bottom field of the above-described 60I signals as an upper circle and lower circle arrayed in two tiers, and further, sets of three fields and the pre-conversion frames thereof are hatched. Note that this notation in FIG. 1 will be applied to subsequent drawings of conventional art and drawings of embodiments of the present invention.
Thus, performing 2-3 pulldown results in the same field being repeated twice at a rate of one field every five fields, and further, as a result thereof, the first frame from the left being converted in the order of top field and bottom field but the third frame from the left being converted in the order of bottom field and top field, the positional relation of the fields being shifted.
As can be understood from the above, 60I signals subjected to 2-3 pulldown differ greatly in nature from normal 60I signals (i.e., generated originally as 60I signals), so handling these equivalently is difficult.
Accordingly, a method can be conceived wherein whether or not 60I signals have been subjected to 2-3 pulldown conversion is detected, and processing is adaptively applied according to the results thereof. For example, in the event of estimating the original 24P signals for 60I signals which have been subjected to 2-3 pulldown (i.e., in the event of performing inverse 2-3 pulldown), there is the need to detect the 2-3 pulldown from the 60I signals in a precise manner.
As for such a 2-3 pulldown detection method, there is a method wherein, as shown in FIG. 2, matching fields and the time interval at which the fields appear is obtained, so as to determine which 2-3 pulldown pattern this falls under.
With the example in FIG. 2, the fields of the input 60I signals (above: top fields, below: bottom fields) are shown along the passage of time t.
That is to say, one sequence is shown configured, in order from the left, of a set of two fields made up of a top field and a bottom field, a set of three fields made up of a top field, a bottom field, and a top field, a set of two fields made up of a bottom field and a top field, a set of three fields made up of a bottom field, a top field, and a bottom field, following which the next sequence configured in the same way as this sequence is shown.
As described above, with 60I signals subjected to 2-3 pulldown, two of the same field is repeated in a set of three fields. Accordingly, in order to detect 2-3 pulldown, fields which match between temporally adjacent top fields or temporally adjacent bottom fields can be detected, and the time interval at which the detected fields appear be obtained, thereby determining which 2-3 pulldown pattern this falls under.
For example, in an ideal case for 2-3 pulldown detection, i.e., in a case wherein the 60I signals are digital signals and proper 2-3 pulldown sequences continue, the difference (pixel difference) between matching fields is zero, as shown in FIG. 3. Accordingly, with such an ideal case, detecting matching fields is relatively simple. Also, in this case, matching fields appear at equal intervals, so determining which 2-3 pulldown pattern this falls under is easy.
However, when we consider actual output television signals or output from player devices such as DVD (Digital Versatile Disk) or the like, the difference between fields which should be matching is not necessarily zero.
That is to say, with television signals, there are cases wherein the main feature of the program is 60I signals subjected to 2-3 pulldown, while commercials are 60I signals not subjected to 2-3 pulldown and so forth, with mixed 60I signals of multiple natures.
With DVD as well, there are cases wherein signals of multiple 60I natures coexist as with television signals, such as a case wherein the main feature of the program is 60I signals subjected to 2-3 pulldown and the menu screen is 60I signals not subjected to 2-3 pulldown. Also, with DVD, there are cases wherein exceptional fields shown in FIG. 4 are inserted in the main feature as well due to authoring and the like, and consequently the proper 2-3 pulldown sequence is not maintained.
In light of the above, input of proper 2-3 pulldown sequence 60I signals is rare, so the difference between fields which should be matching is often not zero. Accordingly, detection among matching fields with little movement is difficult, and consequently, detection of whether or not there has been 2-3 pulldown is difficult.
Now, conventionally, there has been proposed a method for detecting between mating fields using a fixed threshold value for 2-3 pulldown detection.
However, the pixel (brightness) difference values for each pixel between fields which originally should match greatly differ depending on the nature of the input 60I signals, as shown in FIG. 5.
FIG. 5 is a diagram illustrating the absolute values of difference of the pixels between fields, and the frequency thereof.
The example in FIG. 5 shows graphs illustrating the absolute values and the frequency thereof for a case wherein the input 60I signals are digital signals, a case of analog signals, and a base of DVD material such as a menu screen for a DVD or the like, in that order from the top of the drawing. With these graphs, the horizontal axes represent the value of absolute difference, and the vertical axes represent the frequency thereof.
In the case of digital signals, the value of absolute difference in pixels between fields is constantly zero. That is to say, the maximum value of value of absolute difference of pixels between the fields is zero in the case of digital signals.
In the case of analog signals, the maximum value of value of absolute difference of pixels between fields is somewhat greater than with the case of digital signals, due to the effects of A/D (Analog/Digital) conversion and white noise. That is to say, value of absolute difference of pixels between the fields is around 5 in the case of analog signals.
In the case of DVD material, the maximum value of value of absolute difference of pixels between fields is considerably greater than with the case of analog signals, due to the effects of MPEG (Moving Picture Experts Group) encoding distortion and the state of authoring. That is to say, value of absolute difference of pixels between the fields is around 40 in the case of DVD material.
Accordingly, in the event that there is a possibility of such signals with different natures being input as 60I signals, setting the threshold value to be used for 2-3 pulldown low, for example, will result in fields to be determined to be matching in the case of DVD material being erroneously determined to be not matching.
Also, setting the threshold value too high may result in fields which should not be determined to be matching being determined as being matching.    Patent Document 1 Japanese Unexamined Patent Application Publication No. 11-69227